TOTAL REACTIVE NITROGEN (NO(Y)) AS AN INDICATOR OF THE SENSITIVITY OFOZONE TO REDUCTIONS IN HYDROCARBON AND NOX EMISSIONS

For areas in the United States not meeting the federal air quality sta ndard for ozone, an issue of continuing controversy is the emphasis to be placed on controlling nitrogen oxides (NO(x)) in addition to emiss ions of reactive organic gases (ROG). To assess conditions under which ROG or NO(x) controls would be most effective, we have analyzed predi ctions from four studies that represent different locations and meteor ological conditions, distinct chemical inputs, e.g., with or without s ignificant biogenic emissions, and different air quality models. A con sistent association is found between the sensitivity of ozone to reduc tions in ROG versus NO(x) emissions and the simulated total reactive n itrogen (NO(y)) at the time and place of peak ozone. In the studies ex amined, ozone was predicted to be reduced most effectively by ROG cont rols at locations where NO(y) concentrations exceeded a threshhold val ue falling in the range of 10 to 25 ppb, whereas NO(x) controls were p redicted to be more effective where NO(y) concentrations were below th at threshhold. The NO(y) level explains much of the difference in ozon e sensitivity at different locations and provides a basis for comparis on of predicted sensitivity from different models. In contrast, the mo rning concentration ratio of ROG to NO(x) that has been used in the pa st is a less reliable indicator Of 03 sensitivity. Measurement of NO(y ) concentrations along with ozone would assist in empirical testing of model predictions of responses to emission reductions.